1. Field of the Invention
The invention relates to a field device for process instrumentation and, more particularly, to a measurement transducer having an analog output to which a two-wire line for transmitting an analog output signal can be connected.
2. Description of the Related Art
Field devices for process instrumentation are often used in automation technology, where the field devices are used, for example, to record and/or influence process variables and are connected to one another by an automation network for interchanging data. Field devices that record a physical or chemical variable as a process variable are often referred to as measurement transducers because they convert the respective variable into a measured value and output the latter to a superordinate control station, for example, or, in the form of an actual value, to a controller for further processing. Examples of such measurement transducers are measurement transducers for a filling level, mass flow, pressure, temperature, pH or conductivity.
EP 1 192 614 B1 discloses a measurement transducer that converts a physical or chemical measurement variable into an analog output signal that corresponds to the measured value and which can be transmitted on a two-wire line. For this purpose, a suitable sensor for the physical or chemical measurement variable, a downstream analog/digital converter, a computation unit subordinate to the latter and an output circuit that is controlled by the computation unit and can be connected to the two-wire line are provided. The sensor converts the measurement variable into a sensor signal which is digitized in the analog/digital converter and is preprocessed in the computation unit to form a desired value which is used to set the analog output signal on the two-wire line by a control device in the output circuit. The digital desired value, in the form of a predefined current value, is first of all converted into an analog desired current value in the output circuit using a digital/analog converter and is converted into an analog loop current by a downstream control circuit. The actual value of the loop current is recorded using a measuring element for current/voltage conversion. Low-impedance current measuring resistors, which are also referred to as shunts, can be used as simple measuring elements for this purpose. The voltage signal which is generated thereby and is proportional to the actual value of the loop current is supplied to a comparison device for the purpose of forming a control deviation from the analog desired current value and the actual value of the loop current.
A controller comprising an analog controller having an integrated operational amplifier uses the control deviation to determine, for the purpose of correcting the control deviation, an actuating signal for an actuator that is formed by a transistor circuit for setting the loop current. At the same time, the measured analog actual value of the loop current is digitized with the aid of an analog/digital converter and is supplied to the computation unit in the form of a digital actual value. The computation unit is thus able to determine deviations between the digital actual value and the digital desired value and to respond thereto by appropriate tracking of the desired value. If impermissibly large deviations occur, the user of the field device is informed of this. This may be effected via a data interface and/or a display apparatus for transmitting and/or displaying an item of information relating to the deviation which has been determined and thus relating to the measurement error. The data interface is part of the output circuit and enables data communication via the two-wire line according to the Highway Addressable Remote Transducer (HART) protocol.
Data communication according to the HART protocol is performed using continuous-phase frequency modulation with a data transmission rate of 1200 bits/sec. The frequency-modulated current signal, the amplitude of which is supposed to be less than 600 μA, is superimposed on the analog output signal from a 4 to 20 mA interface in a known manner.
DE 10 2007 059 847 A1 discloses a field device having an analog output, in which it is possible to detect a malfunction of the controller or a malfunction of the actuator for the loop current. For this purpose, the actuating signal is monitored for compliance with a predefinable range of values with the aid of a comparator. If a deviation from the range of values occurs, an indication signal signals the detected error state and suitable error-handling measures may be initiated. This may be, for example, the output of an error message or the notification of a superordinate control station of a need for maintenance via the two-wire line and/or the assumption of a safety state by the field device. A safety state is indicated in the analog output signal by a current level that is greater than 22.6 mA or less than 3.5 mA.
One problem when monitoring an analog output circuit for correct operation is represented by the low-pass filter that usually has to be used to smooth the analog output signal. In particular, if a favorable pulse width modulator with a downstream low-pass filter is used for digital/analog conversion, the low-pass filter, which is intended to limit the analog output signal to a bandwidth of less than 25 Hz, constitutes a considerable part of the analog output circuit. Removing the frequency-modulated signal that has been superimposed for data transmission is also problematic when monitoring the analog output current for a correctly set value if a low-pass filter is used for this purpose. Low-pass filters always give rise to a signal delay, with the result that a current value measured at the output for monitoring purposes has a considerable delay with respect to a digital value which is applied to a digital/analog converter of the analog output circuit in order to generate the analog output signal. In addition, low-pass filters usually do not have a constant group delay time, with the result that transient signals in the transition area are output in a corrupted manner. The signal delay in the analog output circuit therefore cannot be easily compensated for by a corresponding delay of the digital value, which is passed to the digital/analog converter, in a monitoring device. One possibility for circumventing the described problems is to exclude time windows with transient signal profiles from the monitoring process. However, a monitoring process would then presuppose a desired value of the output current which is constant over a minimum period of time. However, since it cannot be ensured that this condition is met in a predefined period of time, a maximum period of time in which an error state of the analog output circuit is detected therefore could not be guaranteed.